Battery Storage Calculator

Size home battery storage against solar generation and self-consumption.

About Battery Storage Calculator

Battery capacity is bounded by whichever is smaller: the surplus solar available to store or the evening demand there is to serve. Oversizing beyond either leaves capacity permanently unused. The economics depend on the gap between import price and export payment, so batteries pay back fastest where export rates are lowest.

Formula

Battery Storage

Capacity = min(surplus solar, evening demand ÷ efficiency), with depth of discharge headroom

self-consumption: Share of generated solar used on site rather than exported
round trip efficiency: Losses in charging and discharging, around 10 per cent

Worked example

With the default values below, Battery Storage Calculator returns Recommended Battery Capacity of 8.827. Change any input above and every figure here is recalculated in your browser.

Inputs used

Daily electricity useminimum 0 kWh: 11 kWh
Daily solar generationminimum 0 kWh: 14 kWh
Share of use during daylightaccepts 0–100 %: 35 %
Round trip efficiencyaccepts 30–100 %: 90 %
Import price per kWhminimum 0: 0.28

Results

Recommended Battery Capacity: 8.827 kWh
Evening Demand to Cover: 7.150 kWh
Daily Surplus Solar Available: 10.150 kWh
Annual Import Saving: $730.73
Self-Consumption Achieved: 84.25%

Step by step

1Values used
2Battery Storage
3Recommended Battery Capacity= 8.827 kWh
4Evening Demand to Cover= 7.150 kWh
5Daily Surplus Solar Available= 10.150 kWh
6Annual Import Saving= 730.73
7Self-Consumption Achieved= 84.25

Assumptions and limits

The result is derived only from the 5 inputs listed above. Anything not among them is not part of the calculation.
In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
Figures are rounded for display to 3 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Frequently Asked Questions

How is Battery Storage calculated?

Capacity = min(surplus solar, evening demand ÷ efficiency), with depth of discharge headroom Battery capacity is bounded by whichever is smaller: the surplus solar available to store or the evening demand there is to serve. Oversizing beyond either leaves capacity permanently unused.

Why does Battery Storage matter?

The economics depend on the gap between import price and export payment, so batteries pay back fastest where export rates are lowest.

What values do I need to enter?

This calculator takes 5 inputs: Daily electricity use, Daily solar generation, Share of use during daylight, Round trip efficiency, Import price per kWh. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.